OAKLAND, Calif. — The Energy and Treasury departments released eagerly awaited guidance Thursday to help renewable energy project developers apply for roughly $3 billion in stimulus funds, which experts say will open the market to many technologies that weren’t economically feasible before.

The departments released the guidance, terms of conditions and a sample application, although applications won’t be accepted until next month. The rules is a major step that will spur private sector investment in clean energy and move the U.S. closer to President Barack Obama’s goal of doubling renewable energy capacity in three years, according to Matt Rogers, a Department of Energy (DOE) senior advisor charged with implementing ARRA funding.

“By getting these rules out there and making it clear how to apply we’re hoping this will bring that private capital back from the sidelines and into the market quickly,” Rogers said during a conference call with reporters Thursday.

The tax grants will offset between 10 percent and 30 percent of the project’s cost, depending on the technology type. Construction must begin by the end of 2010 and the projects must be placed into service by 2017 at the latest for certain types of technologies. The Treasury Department expects the program will benefit some 5,000 projects, and seems ready to boost funding from an estimated $3 billion if demand warrants an increase.

Credit termination date and credit percentage, by project type

Under this temporary program, developers who previously qualified for the production tax credit can now opt for the investment tax credit, which is based on the cost of the project, not the amount of electricity to be generated. Those eligible to claim the investment tax credit may then elect to receive a direct payment, rather than having the credit paid over 10 years and based on the amount of electricity generated. Cash grant recipients must agree to give up future tax credits.